1. Field of the Invention
The present invention relates to an optical film, and, in particular, relates to an optical film capable of avoiding optical defects (e.g. wet-out, moire fringes and so on). Further, the present invention relates to an optical film used to the backlight module of a flat panel display.
2. Description of Related Art
The backlight module of the flat panel display and its common structure are very familiar to a person with ordinary skills in the art. Most of the current backlight modules used in the flat panel display employ two light enhancement sheets in order to have better output brightness. However, a troublesome optical coupling effect (i.e. wet-out) more frequently occurs when the two stacked light enhancement sheets get close to each other.
The convention methods for reducing the occurrence of the wet-out let the two optical films physically away from each other as far as possible. That is, the heights of adjacent portions of the two optical films are accordingly modified. U.S. Pat. No. 5,771,328 disclosed an optical film having taller prisms (See numeral reference 56 in its drawings) and shorter prisms (See numeral reference 54 in its drawings). When the optical film is disposed on another one, the physical proximity to another is limited so that the visibility of a wet-out pattern is reduced. However, the taller prisms of the prior art still may be visual as straight lines through the optical film.
In view of above, since the applicant of the present application have found the shortcomings of the foregoing prior art for a long term, he proposes a novel optical film so as to overcome the shortcomings.